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Low Frequency (Electrical Conductivity & Magnetic Susceptibility)

Instruments

Geonics EM31
Geonics EM38

What is measured & how?

A transmitter coil broadcasts an electromagnetic wave of a few kHz that passes through the ground inducing an electric current as it does so. This is turn generates a corresponding magnetic field & the resulting electromagnetic field is re-radiated & detected at the receiver coil. The greater the coil separation the greater the sensitivity of the instrument to deeper features.

Typical targets

Both instruments detect similar features but at different depths in the ground. Metal objects & soils containing metallic debris are easily detected as are variations in soil conductivity due to differences in the amount of water bound up in the soil. They are ideal for the detection of metal working debris & other metallic waste as well as chemical changes in the soil that influence its conductivity. The EM31 has been used very successfully to detect buried voids in dry conditions.

Sampling

Sampling resolution depends on instrument & purpose & the figures here are for guidance only. For area surveys the resolution does not need to exceed the coil spacing in the direction of the long axis of the instrument & no more than half this in the perpendicular direction. For many surveys with the EM31 lines at between 2 & 5m separation are walked with the instrument aligned along each line.

Caveats

This type of instrument relies upon adequate splitting of the return signal by phase so that the inphase & quadrature (out-of-phase) components are meaningful. Instrument set up must be done with care to ensure this is the case. In addition, steel objects possess quadrature susceptibility (high magnetic permeability) which acts in opposition to the effects of electrical conductivity & can make a steel object appear non-conductive.

High Frequency (Radar)

Instruments

GSSI SIR-2000

What is measured & how?

Radar measures the strength of the signal reflected as a radio wave of between 200MHz & 1GHz passes across boundaries between materials of different dielectric permittivity. In practice these include stone, different soils, water & air & any varying combination of these will in theory create the partial reflection of the wave. This reflection is measured for a fixed length of time, e.g. 100ns, & the result digitised to create a series of reflection values at increasing times after transmission equating to successively greater depths into the material.

Typical targets

The sorts of targets most often sought with radar are buried walls & voids, especially below existing structures. It has also been used to locate individual burials in cemeteries & to measure the thickness of materials, including ice. An interesting application that has recently emerged is the detection of persons buried within collapsed buildings through detection of the movements of their chest cavity during breathing. The character of the movements would appear to offer medically-diagnostic information to rescuers.

Sampling

The instrument is invariably used along parallel lines with samples some fraction of a metre apart along each line (e.g. 20 - 30 for archaeological surveys). For these surveys lines are usually 0.5m apart & where maximum detail is required two orthogonal sets of these lines may be surveyed. For other applications, e.g., profiling soils & shallow geological features, the line separation is usually considerably larger, depending upon the scale & variability of the required targets.

Caveats

Radar will not image through metal sheet or through metal mesh with wires closer than the wavelength of radiation. There are some regions where radar survey is prohibited or at least restricted due to the presence of sensitive military installations, prisons, airports or radio telescopes. Care must be taken when using unshielded antennae as any materials (including the surveyor) within similar distances as the targets sought will produce their own radar reflections!